Most of the currently available posterior dynamic stabilization (PDS) devices accommodate bending of the rod. However, if a functional spinal unit (FSU) rotates about a point away from the rod, the rod should elongate and/or compress in addition to bend. Furthermore, a PDS device may limit the motion of an FSU by providing a limit to flexion (i.e., elongation of PDS) or extension (i.e., compression of PDS).
A normal functional spine unit (i.e., two adjacent vertebral segment with a disc in between) has a center of rotation generally near the center of the disc or at least around the vertebral body column. Therefore, any PDS device that is attached to pedicle screws must not only bend but also elongate and compress to allow motion in an FSU.
Furthermore, the need for a PDS device that can elongate and compress becomes more important when a disc replacement prosthetic is implanted, since most disc replacement prosthetics are ball and socket type joints that define the center of vertebral rotation to be near the center of disc. With the center of rotation forced to be in the anterior portion by the disc replacement, the PDS device must elongate and compress to allow vertebral motion.
U.S. Pat. No. 4,790,303 (Steffee) discloses a fastener for securing bone graft between a pair of bone portions. The fastener includes a curved elongated member having a shank for extending into the bone graft and pair of bone portions. The shank has an end portion which receives a force for driving the shank into the pair of bone portions and the bone graft. The shank has a plurality barbs projecting therefrom for resisting movement of the shank relative to the bone graft and the bone portions.
U.S. Pat. No. 5,092,866 (Breard I) discloses an inter-vertebral stabilizer having one or more flexible ligaments. Each flexible ligament can be engaged with two respective vertebrae and/or associated with two retaining elements, such as screws, each of which is suitable for being implanted in a respective vertebra. The present invention also relates to a process, and the associated apparatus, for determining or verifying the tension of such an inter-vertebral stabilizer before it is put into place on the spinal column. This process includes implanting, in each of the vertebrae concerned, a corresponding rigid rod extending outside the patient's body. The rods in each pair of adjacent rods are immobilized in an initial position. If the pain which is to be removed by the stabilizer persists, the process includes modifying the distance between the rods, then in immobilizing the rods in their new relative positions and in repeating the pain test. This cycle of operations is repeated, if necessary, until the pain disappears. The length to be allocated to the ligament is deduced from the distance then attained between the two rods.
U.S. Pat. No. 5,387,213 (Breard II) discloses a surgical implant for connecting two flexible ligaments to vertebrae having an intervertebral stabilizer including an intra-osseous rod having a first end for implanting into a vertebra. The intervertebral stabilizer also includes an extra-osseous head extending outwardly of the vertebrae. The extra-osseous head has two stepped portions, each with a peripheral surface, defining two ligament retaining zones. Each of the two ligament retaining zones has a shoulder, forming an axial ligament abutment on the sides of the ligament retaining zones closest to the first end. The extra-osseous head retains two flexible ligaments spaced from the vertebrae and each other by the shoulders.
U.S. Pat. No. 5,725,582 (Bevan) discloses a surgical implant comprises a hank formed from a single strand of flexible biocompatible material (such as polyester) with at least one bight at each end of the hank and a tail extending from one end, and a crimpable sleeve-like element encircling the overlapping end lengths of the strand. The implant is shown in use for the stabilization of the spine, the bights being applied to hooking members engaged respectively with the lamina of one vertebra and the spinous process of an adjacent vertebra, the strand material being tensioned by pulling the tail before crimping the sleeve-like element.
U.S. Pat. No. 6,436,099 (Drewry I) discloses an apparatus provided to allow for an adjustable length tether for use in the spine and other parts of the body. The tether comprises an artificial strand with an eyelet formed in one end, the other end being looped through the eyelet. The other end is then secured with respect to the eyelet by a crimp, the excess length being cut off after the length of the tether has been given an appropriate tension. Alternatively, the eyelet end may be formed around a grommet. The crimp may be separate from the grommet or a part of the grommet. The mechanism by which the length is adjusted in some cases will take advantage of the shape memory properties of alloys such as nickel-titanium.
WO 2001-45576 (Mulholland) discloses an assembly used for the stabilisation of two adjacent vertebral bodies of the spine. It comprises two pedicle screws, having a threaded shaft with a tapering first end for introduction into the vertebral bodies and a head portion with a second end. It further comprises a flexible longitudinal fulcrum with two portions, which can be disposed transversely to said pedicle screws, and fixed with its end portions to the head portions of said two pedicle screws at a distance x from said second end. The assembly further comprises an elastic ligament which can be disposed transversely to said two pedicle screws and fixed to the head portions of said two pedicle screws at a distance y<x from the end portion of the fulcrum. By introduction of a fulcrum, lying close to the axis of flexion and extension of the spinal motion segment, the posteriorly placed ligament distracts the whole disc, and the fulcrum itself becomes a load-bearing structure. This reduces the load over the disc, and makes it an even distribution of load across the end-plate throughout the range of movement of the motion segment, that is allowed by the implant.
U.S. Pat. No. 6,783,527 (Drewry II) discloses devices, methods and systems for stabilizing at least a portion of the spinal column. Devices include anchors and coupling members for engaging an elongate member. Systems include an elongate member sized to span a distance between at least two vertebral bodies and being at least partially formed of a flexible material. A number of anchors and coupling members are used to secure the elongate member to each of the vertebral bodies. The anchors can be compressed towards one another and the elongate member secured thereto and/or the elongate member can be tensioned to provide corrective forces to the spine.
U.S. Pat. No. 7,018,379 (Drewry III) discloses Devices, methods and systems for stabilizing at least a portion of the spinal column are provided. Devices include anchors and coupling members for engaging an elongate member. Systems include an elongate member sized to span a distance between at least two vertebral bodies and being at least partially formed of a flexible material. A number of anchors and coupling members are used to secure the elongate member to each of the vertebral bodies. The anchors can be compressed towards one another and the elongate member secured thereto and/or the elongate member can be tensioned to provide corrective forces to the spine.
EP Patent No. 0669109 (Dubois) discloses a system to stabilize adjacent vertebrae having a pressure-resistant support body to transfer pressure forces between two screw heads. The strip is of an elastic plastics material with a round cross section to withstand shear forces, fitting into matching drillings to be held on all sides at the support and screw head and to center the support and screw head with each other. The strip is pretensioned to hold the support and screw head together at a support surface round the strip. The elastic material for the support is preferably polyurethane.